Claim and Claim Adjustment Expense Reserves - Schedule of Net Cumulative Paid Claim and Claim Adjustment Expenses (General Liability) (Detail) - CNA Financial [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2007	$ 4,167	$ 4,089	$ 4,355
Total net liability for unpaid claim and claim adjustment expenses	18,249	18,576	18,927	$ 19,117
Property and Casualty Operations [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	14,910	15,363
Property and Casualty Operations [Member] | General Liability [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	4,002
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	2,063
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2007	1,130
Liability for unallocated claim adjustment expenses for accident years presented	55
Total net liability for unpaid claim and claim adjustment expenses	3,248
Property and Casualty Operations [Member] | General Liability [Member] | 2007 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	504	497	492	481	$ 458	$ 413	$ 328	$ 236	$ 130	$ 30
Property and Casualty Operations [Member] | General Liability [Member] | 2008 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	567	560	550	528	473	390	261	129	$ 31
Property and Casualty Operations [Member] | General Liability [Member] | 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	584	557	532	486	392	270	112	$ 33
Property and Casualty Operations [Member] | General Liability [Member] | 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	608	577	530	414	267	139	$ 27
Property and Casualty Operations [Member] | General Liability [Member] | 2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	534	484	389	253	135	$ 27
Property and Casualty Operations [Member] | General Liability [Member] | 2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	417	340	233	127	$ 27
Property and Casualty Operations [Member] | General Liability [Member] | 2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	377	257	135	$ 33
Property and Casualty Operations [Member] | General Liability [Member] | 2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	245	115	$ 29
Property and Casualty Operations [Member] | General Liability [Member] | 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	132	$ 31
Property and Casualty Operations [Member] | General Liability [Member] | 2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 34